At one stage during Australia's run chase against Pakistan, the screen showed 81 required off of 36 balls. And that, in itself, sums up how T20 has completed kicked over the traces of how we view one day cricket.
Back in more sedate times - which is less than a decade ago, the graphic would have been 81 off of 6 overs, and that would have been that. If you'd been in the ground, you'd have probably thought about packing up and leaving to beat the traffic - especially if you factor in the fact that the chasing side were five wickets down.
Now though, T20 has totally rewritten the rules on run chases. Replaces 'balls' with 'overs' is party psychological and a subliminal prompt to the watching millions on TV not to turn over, but partly a recognition that no target is out of reach until the first figure is 'six times plus one' greater than the second figure.
In the early 70's, in the 40 over John Player Sunday League, a first innings score of 150 probably wasn't enough, 170 was considered 'competitive', over 200 was 'challenging' and anything over 220 was pretty much out of reach for the side batting second unless one of two of their batsmen really clicked. Now think that you can transpose those figures almost exactly into the T20 format - that lasts half the time. The only slight caveat being that there were no fielding restrictions in the JPL, though bowlers were limited to 15 yard run ups.
Incidentally, I'm surprised that no one has written a book about the John Player League - or at least the first decade or so of it. Not only have you got the raw statistical data and the revolutionary nature of the tournament at the time it was launched, but there must be a whole wealth of anecdotes surrounding the fact that the games took place mid way through a county game, which might not have even been taking place in the same half of the country. So you'd have a situation where, for example, Kent would be playing a championship game against Somerset at Taunton on the Saturday, have to travel to Headingley for a JPL game on the Sunday, then back to the West Country for the Monday morning resumption of the championship game.